Product formation (stoichiometry) linked to microbial community structure
The microbial community structure at 40-50 d SRT was dominated byBifidobacterium scardovii and supported by Megasphaera andPropionibacterium based on 16S rRNA analysis. This microbial structure could explain the observed product spectrum of propionate and acetate. Bifidobacterium are known for the production of acetic and lactic acid using the phospho-ketolase pathway from glucose using the ‘bifid shunt’, though no propionate production has been as yet associated with Bifidobacterium (Falsen et al., 2015; Pokusaeva, Fitzgerald, & Van Sinderen, 2011; Ventura, Delgado, Milani, O’callaghan, & Van Sinderen, 2016). The microorganism is lacking the key enzymes for the glycolysis and hexose-monophosphate pathway and uses the phospho-ketolase pathway for catabolism (de Vries & Stouthamer, 1967). Possibly, the other observed species Megasphaera cerevisiae and Propionibacterium thoenii capable of consuming the lactate and producing propionate were responsible for the production of propionate in this study (Lanjekar, Marathe, Ramana, Shouche, & Ranade, 2014; Paikt & Glatz, 1997). Potentially, lactate consumption was faster than lactate production at elevated SRT-values (10-20 d and 40-50 d) since no transient lactate was observed. Looking at the direct fermentation of glucose to propionate and acetate a product spectrum of 2:1 (molpropionate:molacetate) can be expected (Gonzalez-Garcia et al., 2017). In this study a product spectrum at 40-50 d SRT of propionate:acetate of 2.05:1 (molpropionate:molacetate) was found, this reflects nicely the proposed stoichiometry by Gonzalez-Garcia et al., (2017). Bifidobacterium was producing lactate and acetate, and subsequently a different microorganism could produce propionate and acetate from lactate.
The microorganism that dominated the process shortly after startupClostridium pasteurianum was also encountered in (J. Tamis et al., 2015). In that work C. pasteurianum dominated the granular biomass over a pH range of 4.5-5.5. Two differences between that study and this study for obtaining C. pasteurianum were that the settling time was set to 2 min and the biogas was continuous diluted by input of fresh N2. C. pasteurianum produced a product spectrum dominated by acetate, butyrate and hydrogen as several studies have reported before for the Clostridium genus (Crabbendam, Neijssel, Tempest, & Amsterdam, 1985; Dabrock, Bahl, & Gottschalk, 1992; Lin et al., 2007; J. Tamis et al., 2015). This study shows that there is still a knowledge gap for obtaining specific product spectrums using granular sludge for the production of VFA from a carbohydrate-rich stream.